For example, a fixed type constant velocity universal joint can be taken as an example of a constant velocity universal joint used as means for transmitting a rotational force from an engine to wheels of an automobile at a constant velocity. The fixed type constant velocity universal joint has a structure in which two shafts, a driving shaft and a driven shaft, are coupled to each other and which is capable of transmitting rotational torque at a constant velocity even when the two shafts form an operating angle. Generally, as disclosed, for example, in Patent Literatures 1 and 2, a Birfield type (BJ) constant velocity universal joint and an undercut-free type (UJ) constant velocity universal joint have been widely known as the above-mentioned fixed type constant velocity universal joint.
For example, as illustrated in FIG. 7, the fixed type constant velocity universal joint of the undercut-free type (UJ) includes: an outer race 3 having an inner surface 1 in which a plurality of guide grooves 2 are equiangularly formed along an axial direction and serving as an outer joint member; an inner race 6 having an outer surface 4 in which a plurality of guide grooves 5 are equiangularly formed in pairs with the guide grooves 2 of the outer race 3 along the axial direction and serving as an inner joint member; a plurality of balls 7 interposed between the guide grooves 2 of the outer race 3 and the guide grooves 5 of the inner race 6, for transmitting torque; and a cage 8 interposed between the inner surface 1 of the outer race 3 and the outer surface 4 of the inner race 6, for retaining the balls 7. In the cage 8, a plurality of window portions 9 for housing the balls 7 are arranged along a circumferential direction.
A groove bottom of each of the guide grooves 2 of the outer race 3 is constituted by an opening-side straight portion 2a (linear portion parallel to the axial direction of the outer race 3) and an interior-side circular-arc portion 2b. A groove bottom of each of the guide grooves 5 of the inner race 6 is constituted by an opening-portion-side circular-arc portion 5a and an interior-side straight portion 5a (linear portion parallel to the axial direction of the inner race 6).
In this case, a center curvature O1 of each of the guide grooves 2 of the outer race 3 and a center O2 of each of the guide grooves 5 of the inner race 6 are offset to opposite sides in the axial direction by equal distances F with respect to a spherical surface center O3 of the inner surface 1 and a spherical surface center O4 of the outer surface 4, respectively.
Further, as illustrated in FIG. 8, the fixed type constant velocity universal joint of the Birfield type (BJ) includes: an outer race 13 having an inner surface 11 in which a plurality of guide grooves 12 are equiangularly formed along an axial direction and serving as an outer joint member; an inner race 16 having an outer surface 14 in which a plurality of guide grooves 15 are equiangularly formed in pairs with the guide grooves 12 of the outer race 13 along the axial direction and serving as an inner joint member; a plurality of balls 17 interposed between the guide grooves 12 of the outer race 13 and the guide grooves 15 of the inner race 16, for transmitting torque; and a cage 18 interposed between the inner surface 11 of the outer race 13 and the outer surface 14 of the inner race 16, for retaining the balls 17. In the cage 18, a plurality of window portions 19 for housing the balls 17 are arranged along a circumferential direction.
In this case, a groove bottom of each of the guide grooves 12 of the outer race 13 and each of the guide grooves 15 of the inner race 16 is constituted only by a circular-arc portion. The center curvature O2 of each of the guide grooves 15 of the inner race 16 and the center curvature O1 of each of the guide grooves 12 of the outer race 13 are offset oppositely to each other in the axial direction by equal distances k and k with respect to a joint center O.
Generally, an operating angle of an automotive fixed type constant velocity universal joint (constant velocity universal joint on a tire side) used for driving of tires of front wheels thereof is set to be low in a straight-advancing state (approximately from 0 to 10 degrees). When an automobile turns, the constant velocity universal joint forms a high angle in accordance with a steering angle. In consideration of general use situations of automobiles, high-angle steering is less-frequently required (garage parking, junctions, and the like), and the constant velocity universal joint is used almost always in the straight-advancing state, that is, at low operating angles. In this context, fuel efficiency improvement of automobiles can be expected by efficiency improvement of the fixed type constant velocity universal joint (reduction of frictional loss) at low operating angles.
Regarding the efficiency improvement of the fixed type constant velocity universal joint, there has been provided a method of realizing a high-efficiency and compact fixed type constant velocity universal joint with use of small-diameter balls 17 and by setting of a track offset amount k′ (k′<k) to be small as illustrated in FIG. 11 (Patent Literatures 1 and 2). As just described, by adoption of the small-diameter balls and the small track offset, differences in movement distance between the inner race 16 and the balls 17 and between the outer race 13 and the balls 17 are reduced. As a result, a relative sliding speed of the balls 17 and the guide grooves 12 of the outer race 13 is reduced, which leads to enhancement in efficiency.
That is, in comparison with the constant velocity universal joint illustrated in FIG. 8 and the constant velocity universal joint in which small track offset is achieved as illustrated in FIG. 11, a wedge angle of the ball 17 is β in the constant velocity universal joint illustrated in FIG. 8, and a wedge angle of the ball 17 is β′ in the constant velocity universal joint illustrated in FIG. 11, the wedge angle β′ being lower than the wedge angle β. As illustrated in FIGS. 9 and 12, a force of pushing out the ball 17 in the axial direction is reduced as indicated by F and F′. With reduction of the force of pushing the ball 17 in the axial direction, a force of the ball 17 by which the cage 18 is pressed against spherical surfaces of the inner and outer races, that is, a spherical surface force is reduced. As a result, frictional loss on the contact portions is reduced, which leads to enhancement in efficiency.
FIG. 10 illustrates a case where the constant velocity universal joint illustrated in FIG. 8 forms an operating angle (40 degrees), and FIG. 13 illustrates a case where the constant velocity universal joint illustrated in FIG. 11 forms an operating angle (40 degrees). In those figures, the line L1 is a trace of contact points of the outer race 13 and the ball 17, and the line L2 is a trace of contact points of the inner race 16 and the ball 17.
Incidentally, six balls 17 are used in the case illustrated in FIG. 8, and eight balls 17 are used in the case illustrated in FIG. 11. Comparison of lengths of the contact point traces and the like of those cases was made, the results of which are shown in Table 1 below.
TABLE 1Length ratio of contact point tracesEight ballsSix ballsInner race11.53Outer race1.061.61Ball diameter ratio11.25Offset ratio11.68
Table 1 shows that the length of the contact point trace in the case of the six balls is longer than that in the case of the eight balls.